Vacuum pump



June 26, 1962 K. F. WESSLING VACUUM PUMP 4 Sheets-Sheet 2 Filed Dec. 2, 1958 INVENTOR. KENNETH F. wessuuc.

BYwvLww, W M.

AIM 7- W June 26, 1962 K. F. WESSLING 3,040,973

, VACUUM PUMP Filed Dec. 2, 1958 4 Sheets-$heet 3 J I24 6/ 1 INVENTOR.

75a 1'05 23 KENNETH Fiwessgmq 79 78 a #MPW4L ATTYS,

June 26, 1962 K. F. WESSLING 3,040,973

VACUUM PUMP Filed Dec. 2, 1958 4 SheetsSheet 4 fATMOSPHERIC INVENTOR. KENNETH FI Wassuufi;

United States 3,040,973 VACUUM PUMP Kenneth F. Wessling, Arlington Heights, ill., assignor to Precision Scientific Company, Chicago, 11]., a corporation of Illinois Filed Dec. 2, 1953, Ser. No. 777,666 6 Claims. (Cl. 230-458) The present invention relates to vacuum pumps and more particularly to a vane type pump capable of porn ing down to pressures on the order of micron or better.

It is the object of the invention to provide a vacuum pump which is highly efficient in operation and which runs quietly, free from the valve clatter characteristic of conventional pumps. It is a more detailed object to provide a vacuum pump having a novel check valve construction which is capable of holding a high vacuum, which is required to cycle at a high repetitive rate, and which has novel means for insuring prompt opening and cushioned closing. It is an object of the invention in one of its aspects to provide a valve construction which is positive and efiicient in operation but which has novel provision for controlled reverse leakage of oil during part of each vacuum pumping cycle.

It is another object of the invention to provide a vacuum pump which is capable of pumping free air at a high volume per unit time as compared to conventional pumps and which is therefore ideally suited for prompt evacuation of systems having a large total volume. It is another object related to the foregoing to provide a pump which is capable of maintaining extremely low pressures on the order of micron or better While consuming an amount of power which is less than that consumed by conventional pumps having the same nominal rating.

It is a further object to provide a vacuum pump which starts without hesitation and which requires no manual priming, enabling use, with a high degree of reliability, in remote or automatically controlled evacuating systems.

It is still another object of the invention to provide a vacuum pump which, after shutdown, may be left connected to an evacuated system without danger that the lubricating and sealing oil will be drawn into the system and without danger of forming a plug of oil between rotor and stator to block rotation of the pump when power is reapplied. In this connection it is an object to provide in a vacuum pump a novel port construction to enable any oil which may accumulate between rotor and stator to be positively disposed of during the first subsequent cycle of rotation.

It is a still further object of the present invention to provide a novel combined trap and reservoir associated with the inlet of the pump for accumulation of oil and for the trapping of any dirt, foreign object or the like which may find its way into the pump inlet. Thus it is the object of the invention to provide a vacuum pump which will operate reliably over long periods of time even in the hands of careless or inexperienced personnel, with the finely machined, sealing surfaces protected against foreign material which may enter the pump It is yet another object of the invention to provide a novel two-stage pump but which may be adapted to any reasonable number of stages and in which the ports between the stages are short and direct with a minimum of idle volume, permitting each of the stages to operate at peak volumetric efliciency.

In one of the aspects of the invention there is an object to provide a vacuum pump which employs a minimum number of parts which are easily machined and quickly assembled together. In this connection it is an object to provide a vacuum pump employing substantially duplicate parts in the successive stages and with the successive stages mechanically phased at equal angular increments, in the present instance /5 of a circular arc or 72. Since the construction is simple and straightforward, pumps of the present design may be manufactured at relatively low cost and may be easily and quickly serviced whenever service may become necessary.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description of the reference to the drawings in which:

FIG. 1 is a perspective showing a pump constructed in accordance with the present invention together with a driving motor.

FIG. 2 is an axial section taken vertically through the pump.

FIG. 3 is a transverse section taken along the line 33 in FIG. 2,

FIG. 3a is a fragmentary section taken along the line Sin-6a in PEG. 2.

FIG. 3'0 is a fragmentary section taken along the line 3b-3b in FIG. 2.

FIG. 4 is a section taken along line 4-4 in FIG. 2.

FIG, 5 is an enlarged top view of the valve member and surrounding tray.

FIG. 6 is a section taken along the line- 6-6 in FIG. 5.

FIG. 7 is a section taken along the line 7-7 in FIG. 3.

FIG. 8 is a fragmentary, developed section taken along the line 3-8 in FIG. 3.

FIG. 9 is a diagram showing the 72 offset between the adjacent pump stators.

FIGS. 10a-10d are a series of stop motion views showing the operation of the check valve when pumping air prior to reaching rated vacuum.

FIG. 11 shows the variations of pressure at the outlet port of the pump for the steps of motion set forth in FIGS. ltla-lOd.

While the invention has been described in connection with a preferred embodiment it will be understood that I do not intend to limit the invention to such embodiment, but intend to cover the alternative and equivalent constructions which may be included Within the spirit and scope of the appended claims.

Turning now to the drawings there is shown a pump 10 constmcted in accordance with the present invention driven by a motor 11 via a belt 12, the pump and motor being secured to a base 13.

While the details of the housing will be discussed at a later point, it may be noted in FIG. 1 that the housing includes an end member 15 which seats a. cup-shaped enclosure 16 having a flange secured by suitable screws 17. Mounted on the top of the end member 15 is an inlet port iii. For providing access, the top surface of the housing 16 is enclosed by an access plate 21 which carries a vent cap '22 thereon of flat disc shape.

As is usual in vacuum pumps, the inlet port it is connected to the system to be evacuated and, upon driving of the pump by the motor, the air or gas withdrawn from the system is discharged from under the vent cap. In

the following discussion it will be assumed that air is being pumped.

The internal construction of the pump is shown in the remainder of the figures and particularly FIG. 2. EX- tending longitudinally through the pump is a shaft 30, one end of which is journaled in the end member 15 and the other end of which is jcurnaled in a bearing or end plate 31. A seal indicated at 32 is interposed between the shaft and the enclosure 16 to prevent escape of the sealing and lubricating oil.

Surrounding the shaft at its left hand end is a first stage or pumping unit 40 having a stator 4-1 in the form of a ring made of cast iron or the like having an irregular outline and a circular opening 42 which is eccentric with respect to the shaft 30. Mounted within the opening &2 is a rotor 43 in the form of a metal disc having accurately ground endfaces, the rotor being in contact with the stator at a terminal region 43a. The rotor 43 is secured to the shaft by a key 44 and is slotted to accommodate radially extending vanes 45, 46 which are spring pressed outwardly against the stator by means of coil spring 47, assembled on a radially extending pin 49' (see FIG. 3a).

For the purpose of admitting air from the inlet 20 into the space between the rotor and stator, an opening is provided in the wall of the end member (FIG. 4). Moreover, for conducting the air, the edges of the opening 42 are grooved or relieved to provide inlet ports 51, 52 which are interconnected by an axially extending passage 53. These inlet ports are preferably machined with a tapering profile as shown in FIG. 3a, with a thickness rela tive to the stator as illustrated in FIG. 8. For discharge of air from the first stage 40, outlet ports are formed in the stator of similar thickness and contour and symmetrically located on the opposite side of the region of contact 43a between stator and rotor. These outlet ports are indicated at 55, 56 and interconnected by a passage 57 (FIG. 8).

The second stage or pumping unit, indicated at 60 in FIG. 3, is substantially identical to the first. It includes a stator 61 having a central opening 62 accommodating a rotor 63 which is in contact with the stator at the terminal region 63a. The rotor is keyed to the shaft 30 by a key 64. Slidably mounted in the rotor are vanes 65, 66 outwardly pressed by spring 67, on a guide pin 69. The second stage has inlet ports 71, 72 formed on the inner surface of the stator and interconnected by a passage 73, as well as outlet ports 75, '76 interconnected by a passage 77.

Interposed between the two stages 40, 60, is a center plate 80 having flat, finely machined side surfaces which bear against the side surfaces of the rotors 43, 63. For providing communication between the outlet port 56 of the first stage and the inlet port 71 of the second, the center plate 80 has a transfer port 8 1 (FIG. 8). To keep the shaft 30 from moving endwise relative to the center plate 80, the shaft is provided with thrust washers 83, 84 which are accommodated in annular recesses 85, S6 machined on each side of the center plate as shown. For the purpose of supplying lubricant between the stages and thereby to insure that the first stage is lubricated by oil leaking into the second stage, the center plate 80 is provided with a bleed opening 88 which interconnects the recesses 85, 86.

Having in mind the construction of the two stages of the pump, mention may be made of the novel manner in which the stages are secured to the end member 15 properly oriented with respect to one another. Thus, in accordance with the invention both of the stators 41, 61 are clamped to the end member 15 by a set of throughbolts received in registering holes which are equally and symmetrically spaced about the pump axis so that when the second pumping stage is rotated through an angle which is equal to the angular spacing between the bolts, the outlet port of the first stage is substantially alined with the inlet port of the second, thereby to minimize the length and volume of the passage 81 which interconnects them. In the present instance the first stator 41 is provided with a series of five holes ala arranged 72 apart and the second stator 61 is provided with a series of holes 61m spaced by an equal amount. When the ptunp is assembled the second stator is rotated through an angle of 72 (see FIG. 9) so that the holes come into register for reception of through-bolts 91 5. This brings the ports 56, 71 into the relation shown in FIG. 8 in which they are both alined with the port 81 formed in the center plate. Corresponding bolt holes are provided in the center plate 80 and the end plate 31 so that when the bolts 91-95 are tightened, the stages become an integral unit with one another and with the end member 15 to which they are secured.

In accordance with one of the important aspects of the present invention, a novel check valve is provided at the outlet port 78 of the second stage, including a leaf spring which in its normal, unstressed state is spaced from the land around the port 78 and which provides intentional leakage of \oil into the pump mechanism during a portion of each pumping cycle. Further in accordance with the invention I employ a composite leaf spring construction including a lower leaf spring and an upper leaf spring arranged flatly face-to-face with the lower leaf spring having an opening therein registered with the outlet port and the upper leaf spring having a combined sealing and cushioning member for sealing the opening in the lower leaf spring to produce a novel and quiet sealing action. Thus, referring to the drawings and particularly to FIGS. 5 and 6, a check valve is provided having a lower leaf spring 101 and an overlying or upper leaf spring 102 arranged flatly adjacent thereto. The leaf springs are mounted at their stationary ends by machine screw 103 or the like. In carrying out the present invention the lower leaf spring 101 is spaced, as shown, above the land 79 which surrounds the outlet port 78 of the second pumping stage, spacing being provided by a spacer 104 surrounding the mounting screw. Registering with the outlet port 78a, and formed in the end of the spring 101 is a circular opening 105 having a diameter which is substantially the same as that of the mouth of the port, which mouth may be desirably formed by chamfering as indicated at 78a. It will be apparent that because of the spacer 104 the lower spring 101., in its unstressed state, is spaced above the land 79 by an amount indicated at 106.

interposed between the leaf springs 101, 102 for sealing the opening 105 in the first leaf spring is a sealing and cushioning disc which is preferably secured to the upper leaf spring 102 by a rivet 111 or the like. A spacer 112 between the springs at their fixed ends separates them an amount which is substantially equal to the thickness of the sealing member 110.

With regard to the materials of construction of the check valve described above, the leaf springs 101, 102 are preferably formed of flat clock spring stock having a thickness of approximately 0.010 inch. The sealing member 110 which may have a thickness of 0.030 inch, is formed of plastic material, preferably a polytetrafluoro ethylene resin widely available under the name of Teflon, although other materials having approximately the same physical characteristics may be employed. Preferably the material should have a hardness within the range of D50 to D65 as measured by the A.S.T.M. method D676. The deformation under load should not exceed 4-8 percent when subjected to a pressure of 1200 p.s.i. in a period of twentyfour hours in accordance with A.S.T.M. method D621. The modulus of elasticity should preferably be on the order of 58,000 psi. in accordance with A.S.T.M. method D638, the above figures being set forth for the guidance of those wishing to employ substitute materials without departing from the present invention.

With regard to the leakage gap indicated at 106 in FIG. 6, my observations show that using conventional lubricating and sealing oil and with the valve submerged to a shallow depth under an oil level 113 this gap may be on the order of 0.060 inch.

It is found that by employing a valve constructed along the lines set forth a number of important and unexpected advantages are derived. Notably, the valve operates quietly free of the clatter which characterizes conventional pumps. It will be appreciated by one skilled in the art that vacuum pumps employed in laboratories are fre quently left connected to an evacuating system for long periods of time and the clatter set up by one or more of the vacuum pumps in constant operation is bothersome and distracting to those working in a laboratory. The

present design of pump, by contrast, produces only slightly more noise than the motor which drives it and a large number of pumps may be operated simultaneously in the same room without raising the noise level to an objectionable degree.

Pumps equipped with the above valve construction are efiicient and are found to make maximum use of the sealing and lubricating oil, permitting high vacuums to be drawn and sustained over long periods of time without any detectable wear of the valve elements and without any change in the operating characteristics. The high efficiency of the valve is particularly surprising in view of the fact that it includes what may be termed a built-in leak. That is to say, when the leaf springs occupy the normal, unstressed position illustrated in FIG. 6, the gap 106 provides sufiicient area for substantial leakage of oil into the port 78. During the portion of the operating cycle when the pressure within the port 78 is at or only slightly below atmospheric pressure, the vacuum is not suliicient to overcome the spring force to effect closure of the valve. Consequently oil is sucked through the gap and down into the second stage of pump. The oil which is thus drawn in effectively seals the rotor to the stator so that there is no leakage or bypassing of air from outlet to inlet. During a subsequent portion of the pumping cycle, the oil which has been drawn in is forcibly expelled and the cycle is repeated.

While the features and advantages of the check valve described above have been found to be important to the operation, the operative phenomena are the subject of continuing investigation. Nevertheless, for further understanding, reference may be made to a series of stop motion views FIGS. a-10d which show the operation of the valve when pumping air and prior to the time that a complete vacuum is drawn. In FIG. 11, and arranged adjacent the stop motion figures, there is shown an approximate plot of pressure variations relative to atmospheric pressure which occur within the outlet port 78, keeping in mind that the purpose of FIG. 11 is to show the direction of pressure variation during the cycle rather than to indicate the magnitude of the variation.

Thus referring to FIG. 10a it will be assumed that the pressure in the port 78 has just gone through atmospheric and is slightly below atmospheric. The force is, however, not sufiicient to draw the leaf springs downwardly into contact with the land surrounding the port. Consequently oil is drawn in through the gap 106 into the port and into contact with the rotor. As the vacuum in the port 78 increases, both of the springs 191, 102 are drawn down tight into contact with the land 79 so that no further oil can enter. As the cycle continues, the active vane in the rotor 63 causes the air between rotor and stator to be compressed so that the pressure in the outlet port 78 rises above atmospheric pressure as shown in FIG. 100. This causes unseating of the leaf springs from the land. Experience shows that the metallic surface on the underside of the spring 1 31 is separated from the metallic surface of the land 79 with ease. However, the pl'astic-to-metal seal between the sealing member 116 and the upper surface of the leaf spring 191 is not quite so readily broken, so that the leaf spring iiil remains in contact with the sealing member during the initial portion of its upward movement. Since the lower spring tends to resist moving upwardly by reason of its resilience, a point in the movement will be reached where the upper spring separates from the lower as shown in FIG. 10d,

whereupon the lower spring is free to assume its initial at rest condition shown in FIG. 10a. The upper spring, however, remains distorted upwardly until the blow oil is complete following which it, too, will be restored to the initial position shown in FIG. 10a in readiness for the ensuing cycle of operation. During the time that the valve is in operation, air bubbles will be seen escaping from under the tips of the springs 101, M2, with the bubbles becoming smaller in size as a high vacuum is drawn. Indicative of proper operation of the valve it is noted in operation that the surface of the oil becomes humped as indicated by the dotted outline 116 in FIGS. 6 and 10a. Such humps indicate that oil is being pumped back and forth by reason of vibration of the leaf springs.

As the vacuum increases, the excursion of the leaf springs will be accordingly reduced, but even under conditions of full rated vacuum the springs continue to vibrate, in a practical pump, through a distance on the order of a inch and observation shows that under such high vacuum conditions oil continues to be pumped in and out of the pump through the port 73 during each cycle of operation.

While the pumping of oil through the outlet port 7s adequately lubrioates the second stage of the pump, it is one of the features of the present construction that means are provided for insuring that lubricant reaches the first stage as well. Thus, as has been previously stated, a separate lubricant-transmitting passage 88 is provided in the center plate 89 so that a portion of the oil lubricating the rotor 63 passes through the opening to lubricate the rotor 4-3 and the thrust washers in the center plate, as well as the associated moving parts.

For permitting evacuation of the recesses surrounding the shaft, passages in the form of grooves 117 (FIG. 3b) are provided in the opposite faces of the center plate interconnecting the transfer port 81 and the recesses 85, 86 formed therein.

In accordance with one of the aspects of the invention means are provided in the present invention for preventing dr-awb'ack of oil into the evacuated system when the pump is turned off but left connected to the system. It often happens during the course of laboratory work Y that a vacuum pump may be turned off without taking the precaution of closing a cock between the pump and the system which has been evac ated. Under such conditions the oil is sucked reversely through the pump and may be sucked right into the system, which is an undesirable condition. Accordingly in the present construction the end member 15 is cored out (FIGS. 2 and 4-) to provide an oil collection space 1125 between the inlet 20 and the port 50 which is formed in the inner wall of the end member 15. Thus the oil which is drawn through the check valve and reversely through the stages of the pump simply collects in the collection space until the oil is at a level below that of the port 78.

Further in accordance with the present invention, the check valve is accommodated in a shallow well or recess formed in the upper surface of the stator 61 thereby to limit the total amount of oil which may be drawn reversely through the pump. In the present instance this function is taken care of by providin a surrounding dam or tray having a mounting portion 121 and a side wall 122. In order to provide clearance about the leaf springs 16], 102, the mounting portion 121 is formed with a rectangular opening 123 as shown in FIG. 5. Arranged under the tray 120 is a gasket 124, the thickness of metal and gasket serving to space the lower spring from the orifice by the 0.060 inch previously mentioned. Thus at one end the tray is held by the mounting screw 103; at the other end a screw 125 is provided.

During normal operation of the pump the oil level 113 is preferably such as to cover the check valve Trill} reliably, say to a depth on the order of to 4; inch. However use of the tray 1'20 insures that the check valve is kept under a reliable depth of oil while nevertheless severely limiting the amount of oil which may be sucked reversely back into the system. As illustrated in FIG. 6, the amount of oil which can be sucked back to the system in the present construction is indicated by the cross hatched area 126. Thus, as soon as sufficient oil has passed through the port 78 to drop the oil level to the upper edge of the tray 120, no further oil can fiow into the tray and thus no further oil can be passed through the port 78. This insures that the reversely sucked oil will be accommodated by the space 115 even though such space is of relatively limited volume.

in accordance with one of the features of the present invention, the oil collection space 1 15 between the inlet port 24) and the port 56 extends substantially below the level of the port d so that any dirt or foreign objects which fall through the inlet port 2% are trapped and cannot pass through the stages of the pump. As illustrated in FIGS. 2 and 4, such foreign material simply tends to collect at l26 from which it maybe periodically flushed. Thus when the pump is in the hands of careless personnel, the chances of the pump being damaged by dirt and foreign objects is minimized.

Further with regard to the tray 12d, my observations show that its use surrounding the valve springs seems to improve the over-all valve operation.

It is one of the features of the present pump that absolute reliability of starting is achieved. It sometimes happens in conventional pumps that oil collects in the space between rotor and stator when the pump stands idle, and this plug of oil prevents the rotor from turning when the driving motor is energized. In the present construction, any oil which may collect between rotor and stator is Simply pushed upwardly toward the elongated outlet ports 75, 76 in the second stage and 55, 56 in the first stage, escaping into such ports prior to the time that the active vane reaches the end of its stroke. This insures that any collected oil is driven out of the pump during the first revolution or so, and it is impossible for a condition of blockage to occur. Also priming is unnecessary. As a result pumps of the present design may be placed in remote or inaccessible locations and may be included as a part of automatically operating apparatus Without fear that a condition of blockage will occur to immobilize the system or to cause damage to the driving motor.

While a two stage pump has been described for purposes of illustrating the present invention, it will be apparent to one skilled in the art that the invention, in some of its aspects, is not limited to a two stage pump but may be extended to pumps having one or more stages while enabling use of substantially identical stators and center plates.

Thus, to add additional stages it is suflicient to rotate each adjacent stage and accompanying center plate through an angle equal to the angle between adjacent throughbolts, namely 72". The substantial identity of rotors and stators of the two or more stages comprising the present design of pump enables worthwhile manufacturing economies. Moreover, the fact that the pump consists of a minimum number of parts, simply machined and simply held together, makes it possible to manufacture the present design of pump at a cost which is less than that of pumps conventionally available. In the event that service becomes necessary, the present pump may be readily disassembled for repair or replacement of parts. As a first step in the disassembly, the oil is drained from the tap 130. The cup-Shaped enclosure 1'6 is loosened by removing the screws 17, and the enclosure is then withdrawn axially to clear the shaft 30. Following this, the throughbolts 91-95 may be unscrewed which enables the stacked stators and their adjacent plates to be separated from one another. Assembly following repair is equally simple.

When access is desired to inspect operation of the check valve 100, the access plate 21 is removed by unscrewing the screws which retain it. It is to be noted that the access plate includes as integral part held thereon by a machine screw, a baffle 131 which prevents escape of droplets of oil through the vent, particularly where the pump is pumping free air at high volume as during the initial stages of evacuating a large-volume system.

I claim as my invention:

1. In a vacuum pump the combination comprising a frame providing an inlet port, a pair of pumping unit-s arranged side-by-side with a separator plate interposed between them, each of said pumping units having a stator and a rotor eccentrically mounted therein and with each of the rotors having slidable vanes bearing against the inner wall of the cooperating stator, each of said stators having an inlet port and an exhaust port and being of identical configuration, said stators being clamped together and to said frame by throughbolts extending therethrough and equally spaced about the periphery thereof, the inlet ports and outlet ports of the stator elements being so angularly spaced that the exhaust port of the first stage is alined with the inlet port of the second stage when one of the stators is rotated with respect to the other through an angle equal to the angle between said throughbolts.

2. In a vacuum pump the combination comprising a frame providing an inlet port, first and second pumping units arranged side-by-side with a separator plate interposed between them, each of said pumping units having a stator and a rotor eccentrically mounted therein and with each of the rotors having slidable vanes bearing against the inner wall of its associated stator, each of said stators having an inlet port and an exhaust port, means for clamping said stators and said separator plate to said frame so that the inlet port of the second unit is in communication with the outlet port of the first unit through an opening provided in said separator plate, means for submerging said units under oil, said separator plate having an auxiliary passage for enabling leakage of oil revcrsely from said second unit to said first unit.

3. In a vacuum pump the combination comprising a frame having a hollow end member providing an inlet port, a plurality of pumping units each having a stator and a sliding-vane rotor eccentrically mounted therein and each having an inlet port and an exhaust port, separator plate means sandwiched between said pumping units and having port means for interconnecting the exhaust port of one unit to the inlet port of the adjacent unit, throughbolts penetrating all of said stators and separator plate means at spaced intervals thereabout for securing said units to said hollow end member, a check valve covering the exhaust port of the final stage, and an enclosure surrounding said pumping units, said enclosure being filled with oil covering said check valve and having a vent for permitting escape of the air passing through said check valve.

4. In a vacuum pump the combination comprising a frame having a hollow end member providing an inlet port, a plurality of pumping units each having a stator and a sliding-vane rotor eccentrically mounted therein and each having an inlet port and an exhaust port, separator plate means sandwiched between said pumping units and having port means for interconnecting the exhaus-t .port of one unit to the inlet port of the adjacent unit, throughbolts penetrating all of said stators and separator plate means at spaced intervals thereabout for valve.

5. In a vacuum pump the combination comprising a frame including an end member of flat disc shape having an inlet port, first and second stator members defining first and second stages, said stator members being arranged flatly side by side and secured to said end member, a ported separator plate interposed between said stator members, a shaft extending through said stator members and said separator plate and having rotors of the sliding vane type mounted thereon, the stators being angularly ofiset so that the exhaust port of the first communicates with the inlet port of the second, a check valve at the exhaust port of the second stator member, an enclosure surrounding said stators and cooperating with said end member for containing a body of oil submerging said check valve, said stators each having at least one peripheral groove in communication with its exhaust port to accommodate any oil which tends to collect in the stages during periods of idleness thereby reducing any tendency toward blockage of the vanes incident to restarting the pump.

6. In a vacuum pump the combination comprising a frame including an end member having an inlet port, first and second stator members defining first and second stages, said stator members being arranged flatly side by side and secured to said end member, a ported separator plate interposed between said stator members, a shaft extending through said stator members and said separator plate and having rotors of the sliding vane type mounted thereon, the stators being so arranged that the exhaust port of the first communicates with the inlet port of the second, a check valve at the exhaust port of the second stator member, an enclosure cooperating With the end member for containing a body of oil submerging said check valve,

the stator members each having at least one peripheral groove communicating with their respective exhaust ports and said end member being of hollow construction defining an oil collection chamber adjacent the inlet port of the pump for accommodation of the oil collected in the pump during periods of idleness.

References Cited in the file of this patent UNITED STATES PATENTS 2,089,630 Teeter Aug. 10, 1937 2,102,346 Wishart Dec. 14, 1937 2,110,107 Drysdale Mar. 1, 1938 2,628,568 Rhine Feb. 17, 1953 2,641,405 Le Valley June 9, 1953 2,750,891 Berry June 19, 1956 2,759,664 Auwarter Aug. 21, 1956 2,816,702 Woodcock Dec. 17, 1957 2,824,687 Osterkamp Feb. 25, 1958 2,831,631 Petersen Apr. 22, 1958 2,877,946 Garrison et a1 Mar. 17, 1959 2,984,186 Livermore et a1 May 16, 1961 

